Coarse frequency synchronization in multicarrier systems

ABSTRACT

A method and apparatus provide coarse frequency synchronization compensating for a carrier frequency deviation from an oscillator frequency in a demodulation system capable of demodulating a signal having a frame structure with at least one useful symbol and a reference symbol which is an amplitude-modulated sequence. A received and down-converted signal undergoes amplitude-demodulation to generate an envelope that is correlated with a predetermined reference pattern to determine the carrier frequency deviation. Finally, the oscillator frequency is controlled based on the carrier frequency deviation. The reference symbol may comprise two identical sequences. In this case, the envelope obtained by the amplitude-demodulation has two portions which are based on the identical sequences. One of the portions of the envelope is correlated with the other one of the portions in order to determine the carrier frequency deviation. The oscillator frequency is controlled based on the determined carrier frequency deviation.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for performing acoarse frequency synchronization compensating for a carrier frequencydeviation from an oscillator frequency in a demodulation system. Inparticular, the present invention relates to such methods and apparatusin a demodulation system for multi-carrier modulation signals, whereinthe multi-carrier modulation (MCM) signals have a frame structurecomprising at least one useful symbol and a reference symbol.

The present invention is particularly useful in a MCM transmissionsystem using an orthogonal frequency division multiplexing (OFDM) fordigital broadcasting.

BACKGROUND OF THE INVENTION

In a multi carrier transmission system (MCM, OFDM), the effect of acarrier frequency offset is substantially more considerable than in asingle carrier transmission system. MCM is more sensitive to phase noiseand frequency offset which occurs as amplitude distortion and intercarrier interference (ICI). The inter carrier interference has theeffect that the subcarriers are no longer orthogonal in relation to eachother. Frequency offsets occur after power on or also later due tofrequency deviation of the oscillators used for downconversion intobaseband. Typical accuracies for the frequency of a free runningoscillator are about ±50 ppm of the carrier frequency. With a carrierfrequency in the S-band of 2.34 GHz, for example, there will be amaximum local oscillator (LO) frequency deviation of above 100 kHz(117.25 kHz). The above named effects result in high requirements on thealgorithm used for frequency offset correction.

DESCRIPTION OF PRIOR ART

Most prior art algorithms for frequency synchronization divide frequencycorrection into two stages. In the first stage, a coarse synchronizationis performed. In the second stage, a fine correction can be achieved. Afrequently used algorithm for coarse synchronization of the carrierfrequency uses a synchronization symbol which has a special spectralpattern in the frequency domain. Such a synchronization symbol is, forexample, a CAZAC sequence (CAZAC=Constant Amplitude ZeroAutocorrelation). Through comparison, i.e. the correlation, of the powerspectrum of the received signal with that of the transmitted signal, thefrequency carrier offset can be coarsely estimated. These prior artalgorithms all work in the frequency domain. Reference is made, forexample, to Ferdinand Claben, Heinrich Meyr, “Synchronization Algorithmsfor an OFDM System for Mobile Communication”, ITG-Fachtagung 130,Codierung für Quelle, Kanal und Übertragung, pp. 105–113, Oct. 26–28,1994; and Timothy M. Schmidl, Donald C. Cox, “Low-Overhead,Low-Complexity [Burst] Synchronization for OFDM”, in Proceedings of theIEEE International Conference on Communication ICC 1996, pp. 1301–1306(1996).

For the coarse synchronization of the carrier frequency, Paul H. Moose,“A Technique for Orthogonal Frequency Division Multiplexing FrequencyOffset Correction”, IEEE Transaction On Communications, Vol. 42, No. 10,October 1994, suggest increasing the spacing between the subcarrierssuch that the subcarrier distance is greater than the maximum frequencydifference between the received and transmitted carriers. The subcarrierdistance is increased by reducing the number of sample values which aretransformed by the Fast Fourier Transform. This corresponds to areduction of the number of sampling values which are transformed by theFast Fourier Transform.

WO 9800946 A relates to a system for a timing and frequencysynchronization of OFDM signals. OFDM training symbols are used toobtain full synchronization in less than two data frames. The OFDMtraining symbols are placed into the OFDM signal, preferably at leastonce every frame. The first OFDM training symbol is produced bymodulating the even-numbered OFDM sub-carriers whereas the odd-numberedOFDM sub-carriers are suppressed. Thus, the first OFDM training symbolis produced by modulating the even-numbered carriers of this symbol witha first predetermined pseudo noise sequence. This results in atime-domain OFDM symbol that has two identical halves since each of theeven-numbered sub-carrier frequencies repeats every half symbolinterval. In case a carrier frequency offset is not greater than asub-carrier bandwidth, the carrier frequency offset can be determinedusing the phase difference between the two halves of the first OFDMtraining symbol. In case the carrier frequency offset can be greaterthan a sub-carrier bandwidth a second OFDM training symbol is used whichis formed by using a second predetermined pseudo noise sequence tomodulate the even-numbered frequencies of this symbol and by using athird predetermined pseudo noise sequence to modulate the odd-numberedcarriers of this symbol. This second OFDM training symbol is used inorder to determine an integer part of the carrier frequency offset. Thisinteger part and a positive or negative fractional part determined fromthe first OFDM training symbol are used for performing the coarsefrequency synchronization. In order to determine the integer part of thecarrier frequency offset, fast Fourier transforms of the two trainingsymbols are required.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide methods andapparatus for performing a coarse frequency synchronization even in thecase of frequency offsets that correspond to a multiple of thesubcarrier distance in a MCM signal. In accordance with a first aspect,the present invention provides a method of performing a coarse frequencysynchronization compensating for a carrier frequency deviation from anoscillator frequency in a demodulation system capable of demodulating asignal having a frame structure, said frame structure comprising atleast one useful symbol and a reference symbol, said reference symbolbeing an amplitude-modulated bit sequence, the method comprising thesteps of:

-   receiving the signal;-   down-converting the received signal;-   performing an amplitude-demodulation of the down-converted signal in    order to generate an envelope; correlating the envelope with a    predetermined reference pattern in order to determine the carrier    frequency deviation; and-   controlling the oscillator frequency based on the carrier frequency    deviation.

In accordance with a second aspect, the present invention provides amethod of performing a coarse frequency synchronization compensating fora carrier frequency deviation from an oscillator frequency in ademodulation system capable of demodulating a signal having a framestructure, the frame structure comprising at least one useful symbol anda reference symbol, the reference symbol being an amplitude-modulatedbit sequence which comprises two identical sequences, the methodcomprising the steps of:

-   receiving the signal;-   down-converting the received signal;-   performing an amplitude-demodulation of the down-converted signal in    order to generate an envelope, the envelope having two portions    which are based on the identical sequences;-   correlating one of the portions of the envelope with another one of    the portions in order to determine the carrier frequency deviation;    and-   controlling the oscillator frequency based on the carrier frequency    deviation.

In accordance with a third aspect, the present invention provides anapparatus for performing a coarse frequency synchronization compensatingfor a carrier frequency deviation from an oscillator frequency, for ademodulation system capable of demodulating a signal having a framestructure, the frame structure comprising at least one useful symbol anda reference symbol, the reference symbol being an amplitude-modulatedbit sequence, the apparatus comprising:

-   receiving means for receiving the signal;-   a down-converter for down-converting the received signal;-   an amplitude-demodulator for performing an amplitude-demodulation of    the down-converted signal in order to generate an envelope;-   a correlator for correlating the envelope with a predetermined    reference pattern in order to determine the carrier frequency    deviation; and-   means for controlling the oscillator frequency based on the carrier    frequency deviation.

In accordance with a fourth aspect, the present invention provides anapparatus for performing a coarse frequency synchronization compensatingfor a carrier frequency deviation from an oscillator frequency, for ademodulation system capable of demodulating a signal having a framestructure, the frame structure comprising at least one useful symbol anda reference symbol, the reference symbol being an amplitude-modulatedbit sequence which comprises two identical sequences, the apparatuscomprising:

-   receiving means for receiving the signal;-   a down-converter for down-converting the received signal;-   an amplitude-demodulator for performing an amplitude-demodulation of    the down-converted signal in order to generate an envelope, the    envelope having two portions which are based on the identical    sequences;-   a correlator for correlating one of the portions of the envelope    with another one of the portions in order to determine the carrier    frequency deviation; and-   means for controlling the oscillator frequency based on the carrier    frequency deviation.

The present invention provides a new scheme for a coarse frequencysynchronization, in particular in MCM systems. The present invention isparticularly useful in systems which use a differential coding andmapping along the frequency axis. In accordance with the presentinvention, the algorithm for the coarse frequency synchronization isbased on a reference symbol which is formed by an amplitude-modulatedsequence. The length of this amplitude-modulated sequence symbol may beless than that of the useful symbol. The algorithm in accordance withthe present invention can be used in the time domain or the frequencydomain. In order to determine a frequency offset, a correlation of thereceived MCM symbol with a predetermined reference pattern is performedin accordance with a first embodiment of the present invention. Inaccordance with a second embodiment of the present invention, thereference symbol comprises at least two identical amplitude-modulatedsequences, wherein a frequency offset is determined based on acorrelation between demodulated portions corresponding to theseidentical sequences. It is preferred to select the mean amplitude of thereference symbol identically to the mean amplitude of the rest of thesignal, i.e. to select all of the samples of the demodulatedamplitude-modulated sequence in the middle of their amplitude range.Care has to be taken that the time constant of an automatic gain control(AGC) is selected to be long enough that the strong signal part of thereference symbol does not excessively influence the automatic gaincontrol signal. Otherwise, the signal occurring after theamplitude-modulated sequence would be strongly attenuated.

According to preferred embodiments of the present invention, theamplitude-modulated sequence of the reference symbol is chosen to be apseudo random bit sequence (PRBS) since such a sequence has goodautocorrelation properties with a distinct correlation maximum in acorrelation signal which should be as wide as possible.

In accordance with preferred embodiments of the present invention, thecoarse frequency synchronization can be performed using theamplitude-modulated sequence after a frame synchronization of a MCMsignal has been accomplished. The inventive algorithm works both in thetime and the frequency domains. Frequency offsets as high as ±10 timesthe subcarrier spacing can be corrected.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, preferred embodiments of the present invention will beexplained in detail on the basis of the drawings enclosed, in which:

FIG. 1 shows a schematic overview of a MCM transmission systemcomprising a coarse frequency synchronization unit in accordance withthe present invention;

FIG. 2 shows a schematic block diagram for illustrating the coarsefrequency synchronization in accordance with the present invention;

FIG. 3 shows a schematic view of a reference symbol comprising twoidentical sequences;

FIG. 4 shows a schematic view of a typical MCM signal having a framestructure;

FIG. 5 shows a block diagram of an embodiment of a coarse frequencysynchronization unit;

FIG. 6 shows a block diagram of another embodiment of a coarse frequencysynchronization unit; and

FIG. 7 shows a block diagram of still another embodiment of a coarsefrequency synchronization unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before discussing the present invention in detail, the mode of operationof a MCM transmission system is described referring to FIG. 1. Althoughthe present invention is explained referring to a MCM system as shown inFIG. 1, it is clear that the present invention can be used in connectionwith different signal transmissions as long as the transmitted signalcomprises useful symbols and reference symbols.

Referring to FIG. 1, at 100 a MCM transmitter is shown thatsubstantially corresponds to a prior art MCM transmitter. A descriptionof such a MCM transmitter can be found, for example, in William Y. Zou,Yiyan Wu, “COFDM: AN OVERVIEW”, IEEE Transactions on Broadcasting, vol.41, No. 1, March 1995.

A data source 102 provides a serial bitstream 104 to the MCMtransmitter. The incoming serial bitstream 104 is applied to abit-carrier mapper 106 which produces a sequence of spectra 108 from theincoming serial bitstream 104. An inverse fast Fourier transform (IFFT)110 is performed on the sequence of spectra 108 in order to produce aMCM time domain signal 112. The MCM time domain signal forms the usefulMCM symbol of the MCM time signal. To avoid intersymbol interference(ISI) caused by multipath distortion, a unit 114 is provided forinserting a guard interval of fixed length between adjacent MCM symbolsin time. In accordance with a preferred embodiment of the presentinvention, the last part of the useful MCM symbol is used as the guardinterval by placing same in front of the useful symbol. The resultingMCM symbol is shown at 115 in FIG. 1 and corresponds to a MCM symbol 160depicted in FIG. 4.

FIG. 4 shows the construction of a typical MCM signal having a framestructure. One frame of the MCM time signal is composed of a pluralityof MCM symbols 160. Each MCM symbol 160 is formed by an useful symbol162 and a guard interval 164 associated therewith. As shown in FIG. 4,each frame comprises one reference symbol 166. The present invention canadvantageously be used with such a MCM signal, however, such a signalstructure being not necessary for performing the present invention aslong as the transmitted signal comprises a useful portion and at leastone reference symbol.

In order to obtain the final frame structure shown in FIG. 4, a unit 116for adding a reference symbol for each predetermined number of MCMsymbols is provided.

In accordance with the present invention, the reference symbol is anamplitude modulated bit sequence. Thus, an amplitude modulation of a bitsequence is performed such that the envelope of the amplitude modulatedbit sequence defines a reference pattern of the reference symbol. Thisreference pattern defined by the envelope of the amplitude modulated bitsequence has to be detected when receiving the MCM signal at a MCMreceiver. In a preferred embodiment of the present invention, a pseudorandom bit sequence having good autocorrelation properties is used asthe bit sequence for the amplitude modulation.

The choice of length and repetition rate of the reference symbol dependson the properties of the channel through which the MCM signal istransmitted, e.g. the coherence time of the channel. In addition, therepetition rate and the length of the reference symbol, in other wordsthe number of useful symbols in each frame, depends on the receiverrequirements concerning mean time for initial synchronization and meantime for resynchronization after synchronization loss due to a channelfade.

The resulting MCM signal having the structure shown at 118 in FIG. 1 isapplied to the transmitter front end 120. Roughly speaking, at thetransmitter front end 120, a digital/analog conversion and anup-converting of the MCM signal is performed. Thereafter, the MCM signalis transmitted through a channel 122.

Following, the mode of operation of a MCM receiver 130 is shortlydescribed referring to FIG. 1. The MCM signal is received at thereceiver front end 132. In the receiver front end 132, the MCM signal isdown-converted and, furthermore, a analog/digital conversion of thedown-converted signal is performed.

The down-converted MCM signal is provided to a symbol frame/carrierfrequency synchronization unit 134.

A first object of the symbol frame/carrier frequency synchronizationunit is to perform a frame synchronization on the basis of theamplitude-modulated reference symbol. This frame synchronization isperformed on the basis of a correlation between theamplitude-demodulated reference symbol and a predetermined referencepattern stored in the MCM receiver.

A second object of the symbol frame/carrier frequency synchronizationunit is to perform a coarse frequency synchronization of the MCM signal.To this end, the symbol frame/carrier frequency synchronization unit 134serves as a coarse frequency synchronization unit for determining acoarse frequency offset of the carrier frequency caused, for example, bya difference of the frequencies between the local oscillator of thetransmitter and the local oscillator of the receiver. The determinedfrequency is used in order to perform a coarse frequency correction. Themode of operation of the coarse frequency synchronization unit isdescribed in detail referring to FIGS. 2 and 3 hereinafter.

As described above, the frame synchronization unit 134 determines thelocation of the reference symbol in the MCM signal. Based on thedetermination of the frame synchronization unit 134, a reference symbolextracting unit 136 extracts the framing information, i.e. the referencesymbol, from the MCM signal coming from the receiver front end 132.After the extraction of the reference symbol, the MCM signal is appliedto a guard interval removal unit 138. The result of the signalprocessing performed hereherto in the MCM receiver are the useful MCMsymbols.

The useful MCM symbols output from the guard interval removal unit 138are provided to a fast Fourier transform unit 140 in order to provide asequence of spectra from the useful symbols. Thereafter, the sequence ofspectra is provided to a carrier-bit mapper 142 in which the serialbitstream is recovered. This serial bitstream is provided to a data sink144.

Following, the mode of operation of the coarse frequency synchronizationunit will be described in detail referring to FIGS. 2 and 3. As it isshown in FIG. 2, the output of the receiver front end 132 is connectedto an analog/digital converter 200. The down-converted MCM signal issampled at the output of the analog/digital converter 200 and is appliedto a frame/timing synchronization unit 202. In a preferred embodiment, afast running automatic gain control (AGC) (not shown) is providedpreceding the frame/timing synchronization unit in order to eliminatefast channel fluctuations. The fast AGC is used in addition to thenormally slow AGC in the signal path, in the case of transmission over amultipath channel with long channel impulse response and frequencyselective fading. The fast AGC adjusts the average amplitude range ofthe signal to the known average amplitude of the reference symbol.

As described above, the frame/timing synchronization unit uses theamplitude-modulated sequence in the received signal in order to extractthe framing information from the MCM signal and further to remove theguard intervals therefrom. After the frame/timing synchronization unit202 it follows a coarse frequency synchronization unit 204 whichestimates a coarse frequency offset based on the amplitude-modulatedsequence of the reference symbol of the MCM signal. In the coarsefrequency synchronization unit 204, a frequency offset of the carrierfrequency with respect to the oscillator frequency in the MCM receiveris determined in order to perform a frequency offset correction in ablock 206. This frequency offset correction in block 206 is performed bya complex multiplication. The output of the frequency offset correctionblock 206 is applied to the MCM demodulator 208 formed by the FastFourier Transform unit 140 and the carrier-bit mapper 142 shown in FIG.1.

In order to perform the inventive coarse frequency synchronization, ineither case, an amplitude-demodulation has to be performed on apreprocessed MCM signal. The preprocessing may be, for example, thedown-conversion and the analog/digital conversion of the MCM signal. Theresult of the amplitude-demodulation of the preprocessed MCM signal isan envelope representing the amplitude of the MCM signal.

For the amplitude demodulation, a simple alpha_(max+)beta_(min−) methodcan be used. This method is described for example in Palacherla A.:DSP-μP Routine Computes Magnitude, EDN, Oct. 26, 1989; and Adams, W. T.,and Bradley, J.: Magnitude Approximations for MicroprocessorImplementation, IEEE Micro, Vol. 3, No. 5, October 1983.

It is clear that amplitude determining methods different from thedescribed alpha_(max+) beta_(min−) method can be used. Forsimplification, it is possible to reduce the amplitude calculation to adetection as to whether the current amplitude is above or below theaverage amplitude. The output signal then consists of a −1/+1 sequencewhich can be used to determine a coarse frequency offset by performing acorrelation. This correlation can easily be performed using a simpleintegrated circuit (IC).

In addition, an oversampling of the signal received at the RF front endcan be performed. For example, the received signal can be expressed withtwo times oversampling.

In accordance with a first embodiment of the present invention, acarrier frequency offset of the MCM signal from an oscillator frequencyin the MCM receiver is determined by correlating the envelope obtainedby performing the amplitude-demodulation as described above with apredetermined reference pattern.

In case there is no frequency offset, the received reference symbol r(k)will be: $\begin{matrix}{{r(k)} = {{S_{AM}(k)} + {n(k)}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$wherein n(k) designates “additive Gaussian noise” and SAM denotes the AMsequence which has been sent. In order to simplify the calculation theadditive Gaussian noise can be neglected. It follows: $\begin{matrix}{{r(k)} \cong {S_{AM}(k)}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

In case a constant frequency offset ?f is present, the received signalwill be: $\begin{matrix}{{\overset{\sim}{r}(k)} = {{S_{AM}(k)} \cdot {\mathbb{e}}^{{j2{\pi\Delta}}\;{fkT}_{MCM}}}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

Information regarding the frequency offset is derived from thecorrelation of the received signal r(k) with the AM sequence S_(AM)which is known in the receiver: $\begin{matrix}{{\sum\limits_{k = 1}^{\frac{L}{2}}{{\overset{\sim}{r}(k)} \cdot {S_{AM}^{*}(k)}}} = {\sum\limits_{k = 1}^{\frac{L}{2}}{{{S_{AM}(k)}}^{2}{\mathbb{e}}^{{j2{\pi\Delta}}\;{fkT}_{MCM}}}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

Thus, the frequency offset is: $\begin{matrix}{{\Delta\; f} = {{\frac{1}{2\pi\; T_{MCM}}{\arg\left( {\sum\limits_{k = 1}^{\frac{L}{2}}{{r(k)} \cdot {S_{AM}^{*}(k)}}} \right)}} - {\frac{1}{2\pi\; T_{MCM}}{\arg\left( {\sum\limits_{k = 1}^{\frac{L}{2}}{{S_{AM}(k)}}^{2}} \right)}}}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

Since the argument of ‥S_(AM)(k)|² is zero the frequency offset is:$\begin{matrix}{{\Delta\; f} = {\frac{1}{2\pi\; T_{MCM}}{\arg\left( {\sum\limits_{k = 1}^{\frac{L}{2}}{{\overset{\sim}{r}(k)} \cdot {S_{AM}^{*}(k)}}} \right)}}} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$

In accordance with a second embodiment of the coarse frequencysynchronization algorithm in accordance with the present invention, areference symbol comprising at least two identical sequences 300 asshown in FIG. 3 is used. FIG. 3 shows the reference symbol of a MCMsignal having two identical sequences 300 of a length of L/2 each. Ldesignates the number of values of the two sequences 300 of thereference symbol.

As shown in FIG. 3, within the amplitude-modulated sequence, there areat least two identical sections devoted to the coarse frequencysynchronization. Two such sections, each containing L/2 samples, areshown at the end of the amplitude-modulated sequence in FIG. 3. Theamplitude-modulated sequence contains a large number of samples. For anon-ambiguous observation of the phase, only enough samples to contain aphase rotation of 2p should be used. This number is defined as L/2 inFIG. 3.

Following, a mathematical derivation of the determination of a carrierfrequency deviation is presented. In accordance with FIG. 3, thefollowing equation applies for the two identical sequences 300:$\begin{matrix}{{s\left( {0 < k \leq \frac{L}{2}} \right)} \equiv {s\left( {\frac{L}{2} < k \leq L} \right)}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

If no frequency offset is present, the following equation 8 will be metby the received signal: $\begin{matrix}{{r\left( {k + \frac{L}{2}} \right)} \equiv {{r(k)}\mspace{14mu} 0} < k \leq \frac{L}{2}} & \left( {{Eq}.\mspace{14mu} 8} \right)\end{matrix}$r(k) designates the values of the identical sequences. k is an indexfrom one to L/2 for the respective samples.

If there is a frequency offset of, for example, ?f, the received signalis: $\begin{matrix}{{\overset{\sim}{r}(k)} = {{r(k)} \cdot {\mathbb{e}}^{{j2{\pi\Delta}}\;{fkT}_{MCM}}}} & \left( {{Eq}.\mspace{14mu} 9} \right) \\{{\overset{\sim}{r}\left( {k + \frac{L}{2}} \right)} = {{r(k)} \cdot {\mathbb{e}}^{{j2{\pi\Delta}}\;{f{({k + \frac{L}{2}})}}T_{MCM}}}} & \left( {{Eq}.\mspace{14mu} 10} \right)\end{matrix}${tilde over (r)} (k) designates sample values of the received portionwhich are based on the identical sequences. Information regarding thefrequency offset is derived from the correlation of the received signal{tilde over (r)} (k+L/2) with the received signal {tilde over (r)} (k).This correlation is given by the following equation: $\begin{matrix}{{\sum\limits_{k = 1}^{\frac{L}{2}}{{{\overset{\sim}{r}}^{*}\left( {k + \frac{L}{2}} \right)}{\overset{\sim}{r}(k)}}} = {\sum\limits_{k = 1}^{\frac{L}{2}}{{{r(k)}}^{2}{\mathbb{e}}^{{- {j2{\pi\Delta}}}\; f\frac{L}{2}T_{MCM}}}}} & \left( {{Eq}.\mspace{14mu} 11} \right)\end{matrix}${tilde over (r)}* designates the complex conjugate of the sample valuesof the portion mentioned above.

Thus, the frequency offset is $\begin{matrix}{{\Delta\; f} = {{\frac{1}{2\pi\frac{L}{2}T_{MCM}}{\arg\left( {\sum\limits_{k = 1}^{\frac{L}{2}}{{\overset{\sim}{r}\left( {k + \frac{L}{2}} \right)} \cdot {{\overset{\sim}{r}}^{*}(k)}}} \right)}} - {\frac{1}{2\pi\frac{L}{2}T_{MCM}}{\arg\left( {\sum\limits_{k = 1}^{\frac{L}{2}}{{\overset{\sim}{r}(k)}}^{2}} \right)}}}} & \left( {{Eq}.\mspace{14mu} 12} \right)\end{matrix}$

Since the argument of |r(k)|² equals zero, the frequency off-set becomes$\begin{matrix}{{\Delta\; f} = {\frac{1}{2\pi\frac{L}{2}T_{MCM}}{\arg\left( {\sum\limits_{k = 1}^{\frac{L}{2}}{{{\overset{\sim}{r}\left( {k + \frac{L}{2}} \right)} \cdot {\overset{\sim}{r}}^{*}}(k)}} \right)}}} & \left( {{Eq}.\mspace{14mu} 13} \right)\end{matrix}$

Thus, it is clear that in both embodiments, described above, thefrequency position of the maximum of the resulting out-put of thecorrelation determines the estimated value of the offset carrier.Furthermore, as it is also shown in FIG. 2, the correction is performedin a feed forward structure.

An apparatus for performing the coarse frequency synchronization using areference symbol having two identical sections of the length of L/2 eachwhich has been described above is shown in FIG. 5.

Also shown in FIG. 5 is the frame/timing synchronization unit 202. Ascan be seen from FIG. 5, a unit 400 for per-forming a fast automaticgain control (time constant<MCM symbol duration) can be providedpreceding the frame/timing synchronization unit. The output of theframe/timing synchronization unit 202 is connected to an extracting unit402 which is operable to extract the last L samples from the referencesymbol. The output of the extracting unit 402 is connected to ademultiplexer 404 which recovers the two identical sections having thelength of L/2 each from the L samples. The identical sections areapplied to a correlator 406 which performs the correlation as describedabove.

The output of the correlator 406 is connected to an operation unit 408for performing an argument operation on the output signal of thecorrelator 406. The output of the operation unit 408 is connected to amultiplier 410 which multiplies the output by ½π(L/2)T_(MCM)). A furtheroperation unit 412 for performing a e^(−j(πΔfT) ^(MCM) ^(/L)) operationis provided in order to derive the frequency shift for the whole MCMsymbol from the frequency shift determined for the portion having thelength of L, i.e. the identical sections 300 shown in FIG. 3.

In case of a channel with strong reflections, for example due to a highbuilding density, the correlations described above might be insufficientfor obtaining a suitable coarse frequency synchronization. Therefore, inaccordance with a third embodiment of the present invention,corresponding values of the two portions which are correlated inaccordance with a second embodiment, can be weighting with correspondingvalues of stored predetermined reference patterns corresponding to saidtwo identical sequences of the reference symbol. This weighting canmaximize the probability of correctly determining the frequency offset.The mathematical description of this weighting is as follows:$\begin{matrix}{{\Delta\; f} = {\frac{1}{2\pi\frac{L}{2}T_{MCM}}{\arg\left( {\sum\limits_{k = 1}^{\frac{L}{2}}{\left\lbrack {{\overset{\sim}{r}\left( {k + \frac{L}{2}} \right)} \cdot {{\overset{\sim}{r}}^{*}(k)}} \right\rbrack \cdot \left\lbrack {{S_{AM}(k)}{S_{AM}^{*}\left( {k + \frac{L}{2}} \right)}} \right\rbrack}} \right)}}} & \left( {{Eq}.\mspace{14mu} 14} \right)\end{matrix}$

S_(AM) designates the amplitude-modulated sequence which is known in thereceiver, and S*_(AM) designates the complex conjugate thereof.

If the above correlations are calculated in the frequency domain, theamount of $\begin{matrix}{\sum\limits_{k = 1}^{\frac{L}{2}}{\left\lbrack {{\overset{\sim}{r}\left( {k + \frac{L}{2}} \right)} \cdot {{\overset{\sim}{r}}^{*}(k)}} \right\rbrack \cdot \left\lbrack {{S_{AM}(k)}{S_{AM}^{*}\left( {k + \frac{L}{2}} \right)}} \right\rbrack}} & \left( {{Eq}.\mspace{14mu} 15} \right)\end{matrix}$is used rather than the argument. This amount is maximized as a functionof a frequency correction. The position of the maximum determines theestimation of the frequency deviation. As mentioned above, thecorrection is performed in a feed forward structure.

A block diagram of an apparatus for performing the coarse frequencysynchronization in accordance with the third embodiment of the presentinvention is shown in FIG. 6. Blocks 400, 202, 402, 404 and 406 shown inthe left branch of FIG. 6 correspond to the respective blocks in FIG. 5.In the right branch of FIG. 6, the preparation of the known AM sequenceis shown. The known AM sequence is read from a memory 420 and applied toan extracting unit 422 which extracts the last L samples therefrom. Theoutput of the extracting unit 422 is connected to a demultiplexer 424having one input and two outputs in order to recover the identicalsections having a length of L/2 each. Both outputs of the demultiplexerare connected with a correlator 426 which performs a correlation betweenthe two identical sections.

A multiplier 428 is provided which multiplies the output of thecorrelator 406 by the output of the correlator 426. The output of themultiplier 428 is connected to an argument operation unit 408. Theoutput of the multiplier is applied to an argument operation unit 408, amultiplier 410 and an operation unit 412 in sequence. The mode ofoperation of these units corresponds to that of the corresponding unitswhich are shown in FIG. 5.

A alternative structure of an apparatus for performing the coarsefrequency synchronization in accordance with the third embodiment of thepresent invention in the frequency domain is shown in FIG. 7. As shownin FIG. 7, a fast Fourier transform unit 440 is provided between thedemultiplexer 404 and a correlator 442, and a fast Fourier transformunit 444 is provided between the demultiplexer 424 and a correlator 426.The outputs of the correlators 442 and 446 are connected to a multiplier445. The output of the multiplier 445 is connected to a maximumsearching unit 447. Finally, a unit 448 for performing a e^(−j(πΔfT)^(MCM) ^(/L)) operation is provided. The output of this unit 448represents the output of the coarse frequency synchronization device.

In case of performing the coarse frequency synchronization in thefrequency domain it is possible to make use of the existing FFT at thebeginning of the detection for the coarse frequency synchronizationrather than providing an additional fast Fourier transform unit.

Following the course frequency synchronization described above, a finefrequency synchronization can be performed in case such a fine frequencysynchronization is useful.

What is claimed is:
 1. A method of performing a coarse frequencysynchronization compensation for a carrier frequency deviation from anoscillator frequency in a demodulation system capable of demodulating asignal having a frame structure, said frame structure comprising atleast one useful symbol and a reference symbol, said reference symbolbeing an amplitude-modulated bit sequence which comprises two identicalsequences, said method comprising the steps of: receiving said signal;down-converting said received signal; performing anamplitude-demodulation of the down-converted signal in order to generatean envelope, said envelope having two portions which are based on saididentical sequences; correlating one of said portions of said envelopewith another one of said portions in order to determine said carrierfrequency deviation; and controlling said oscillator frequency based onsaid carrier frequency deviation: wherein said correlating step furthercomprises weighting of corresponding values of said two portions withcorresponding values of said two sequences.
 2. The method of claim 1,wherein said carrier frequency deviation is determined as follows:${\Delta\; f} = {\frac{1}{2\pi\frac{L}{2}T_{MCM}}{\arg\left( {\sum\limits_{k = 1}^{\frac{L}{2}}{\left\lbrack {{\overset{\sim}{r}\left( {k + \frac{L}{2}} \right)} \cdot {{\overset{\sim}{r}}^{*}(k)}} \right\rbrack \cdot \left\lbrack {{S_{AM}(k)}{S_{AM}^{*}\left( {k + \frac{L}{2}} \right)}} \right\rbrack}} \right)}}$wherein {tilde over (r)} designates values of said portions; {tilde over(r)}* designates the complex conjugate of said values of said portions;T_(MCM) designates the duration of said useful symbol; k designates anindex; L designates the number of values of said two sequences of saidreference symbol; S_(AM) designates values of said identical sequences;and S*_(AM) designates the complex conjugate of said values of saididentical sequences.
 3. The method according to claim 1, wherein saidsignal is an orthogonal frequency division multiplex signal.
 4. Themethod according to claim 1, further comprising the step of performing afast automatic gain control of said received down-converted signal priorto the step of performing said amplitude-demodulation.
 5. The methodaccording to claim 1, wherein the step of performing saidamplitude-demodulation comprises the step of calculating an amplitude ofsaid signal using the alpha_(max+)beta_(min−) method.
 6. The methodaccording to claim 1, further comprising the steps of samplingrespective amplitudes of said received down-converted signal andcomparing said sampled amplitudes with a predetermined threshold inorder to generate a bit sequence in order to perform saidamplitude-demodulation.
 7. The method according to claim 6, wherein thestep of sampling respective amplitudes of said received down-convertedsignal further comprises the step of performing an over-sampling of saidreceived down-converted signal.
 8. An apparatus for performing a coarsefrequency synchronization compensation for a carrier frequency deviationfrom an oscillator frequency, for a demodulation system capable ofdemodulating a signal having a frame structure, said frame structurecomprising at least one useful symbol and a reference symbol, saidreference symbol being an amplitude-modulated bit sequence whichcomprises two identical sequences, said apparatus comprising: receivingmeans for receiving said signal; a down-converter for down-convertingsaid received signal; an amplitude-demodulator for performing anamplitude-demodulation of said down-converted signal in order togenerate an envelope, said envelope having two portions which are basedon said identical sequences; a correlator for correlating one of saidportions of said envelope with another one of said portions in order todetermine said carrier frequency deviation; and means for controllingsaid oscillator frequency based on said carrier frequency deviation;wherein said correlator comprises means for weighting of correspondingvalues of said two portions with corresponding values of said twosequences.
 9. The apparatus of claim 8, further comprising means fordetermining said carrier frequency deviation as follows:${\Delta\; f} = {\frac{1}{2\pi\;\frac{L}{2}T_{MCM}}{\arg\left( {\sum\limits_{k = 1}^{\frac{L}{2}}{\left\lbrack {{r\left( {k + \frac{L}{2}} \right)} \cdot {{\overset{\sim}{r}}^{*}(k)}} \right\rbrack \cdot \left\lbrack {{S_{AM}(k)}{S_{AM}^{*}\left( {k + \frac{L}{2}} \right)}} \right\rbrack}} \right)}}$wherein {tilde over (r)} designates values of said portions; {tilde over(r)}* designates the complex conjugate of said values of said portions;T_(MCM) designates the duration of said useful symbol; k designates anindex; L designates the number of values of said two sequences of saidreference symbol; S_(AM) designates values of said identical sequences;and S*_(AM) designates the complex conjugate of said values of saididentical sequences.
 10. The apparatus according to claim 8, whereinsaid signal is an orthogonal frequency division multiplexed signal. 11.The apparatus according to claim 8, further comprising means forperforming a fast automatic gain control of said received down-convertedsignal preceding said amplitude-demodulator.
 12. The apparatus accordingto claim 8, wherein said amplitude-demodulator comprises means forcalculating an amplitude of said signal using an alpha_(max+)beta_(min−) method.
 13. The apparatus according to claim 8, furthercomprising means for sampling respective amplitudes of saiddown-converted signal, wherein said amplitude-demodulator comprisesmeans for comparing said sampled amplitudes with a predeterminedthreshold in order to generate a bit sequence.
 14. The apparatusaccording to claim 13, wherein said means for sampling comprises meansfor over-sampling said down-converted signal.